Transition metal-catalyzed oxygen transfer reactions are some of the most important reaction types in the chemical industry. At the forefront is the partial oxidation of hydrocarbons with molecular oxygen. Molecular oxygen in the form of atmospheric oxygen is by far the cheapest oxidizing agent. In the base state, oxygen is present as a paramagnetic triplet and is relatively unreactive. As a result of interaction with the catalyst surface, the oxygen is activated by coordination to the catalyst surface, electron transfer, dissociation of the oxygen molecule and incorporation of oxygen atoms into the oxidic crystal lattice. Lattice oxygen can be incorporated into an activated hydrocarbon by a nucleophilic addition; the oxygenated products are desorbed from the catalyst surface. The reduced catalyst surface is subsequently reoxidized by gas phase oxygen.
The oxidation of n-butane to maleic anhydride is an example of an industrially employed reaction, which requires, in a single operation, the withdrawal of eight hydrogen atoms, the incorporation of three oxygen atoms and the transfer of 14 electrons (see, for example, Millet J. M. M., Topics on Catalysis 2006, Vol. 38, P. 83 to 92).
It is evident from what has been stated that this demanding reaction requires a catalyst with good electronic, oxygen and/or proton conductivity. More particularly, the reaction rate for the selective oxidation of hydrocarbons can be limited by electronic and/or ionic transport properties, i.e., for example, by the transfer of electrons or holes (in other words the oxidation or reduction of the transition metal ions of the catalyst), of hydrogen or oxygen atoms from the catalyst surface to the substrate (the hydrocarbon) and vice versa, and hence places high demands on the catalyst.
One of the great challenges is the development of new catalysts with improved selectivity and/or higher conversion. The development of new catalysts, however, is very complex. One reason for this is that most of the industrial catalysts are multielement oxides which, as well as the catalytically active transition metal oxide, also comprise what are called promoters, which enhance the catalyst action and/or improve the selectivity. A shortening of the development times for new catalysts is desirable.
It is an object of the invention to provide catalysts for the gas phase oxidation of organic hydrocarbons with improved selectivity and/or higher conversion. It is another object of the invention to provide a process for optimizing a catalyst for the gas phase oxidation of organic hydrocarbons.
The invention relates to a catalyst for the gas phase oxidation of organic hydrocarbons, comprising a multielement oxide which comprises at least one transition metal, wherein the catalyst has a charge transport activation energy Ec at a temperature of 375 to 425° C., especially about 400° C., of less than 0 kJ/mol (negative charge transport activation energy).